1. Field of the Invention
The present invention relates to a mobile station and a method of switching connection destinations.
2. Description of the Related Art
In a mobile communication system, a mobile station generally performs an operation of switching connection destination radio base stations when the quality of radio communication with a current radio base station gets worse. The operation of switching the connection destination radio base station is referred to as handover. FIG. 1 is a sequence diagram illustrating an operation of a mobile communication system performing handover.
Mobile station 205 is performing data communication with radio base station 202. Here, mobile station 205 measures the quality of radio communication with radio base station 202 based on a signal received from mobile station 202. Examples of the quality of radio communication include the reception level of the received signal, the ratio of the reception level to the noise level (hereinafter, referred to as Signal to Noise (SN) ratio), etc. Examples of the signal for measuring the quality of radio communication include a piece of notification information, a data frame, an acknowledgment packet in response to a data frame sent from mobile station 205, etc.
Mobile station 205 starts handover when the quality of radio communication is below a predetermined threshold (step A).
Mobile station 205 sends a search packet for searching other neighboring radio base stations using radio broadcast communication (step B). Then, mobile station 205 receives a search response packet from one or more of the neighboring radio base stations (step C).
Mobile station 205 determines, based on the search response packet, the quality of radio communication with the radio base station, which sent the search response packet, and remembers a table in which the qualities of radio communication are arranged according to radio base stations. Mobile station 205 selects a radio base station having the best quality of radio communication by comparing the qualities of radio communication in the table. Then, mobile station 205 switches a connection destination to the selected radio base station by notifying a connection switching request to the selected radio base station.
In this case, the radio base station acting as the connection destination is determined based on the qualities of radio communication compared by mobile station 205. However, in real-time communication processing of voices and dynamic images, when the connection destination is switched based on only the qualities of radio communication compared by mobile station 205, communication with the switched radio base station is sometimes not continued. For example, when the available bandwidth of the switched radio base station is subjected to excessive traffic load due to congestion, communication is sometimes not continued. Therefore, the following process is performed.
When the search response packet is received (step C), mobile station 205 notifies currently-communicating radio base station 202 of the table, in which the qualities of radio communication are arranged according to the radio base stations, as search result information (step D). Radio base station 202 checks the state of available bandwidths of the radio base stations included in the search result information, and notifies mobile station 205 of a radio base station having the best available bandwidth as connection candidate information (step E).
Mobile station 205 notifies a connection switching request to the mobile station corresponding to the notified connection candidate information (hereinafter, referred to as radio base station 203) (step F). When the connection switching request is received, radio base station 203 notifies mobile station 205 that connection switching is completed, thereby completing the handover.
Technologies capable of performing such handover include for example a mobile station disclosed in Document 1 (Japanese Laid-Open Patent Application 2001-95028).
According to the technology capable of performing handover, mobile station 205 sends search result information to currently-communicating radio base station 202 and receives connection candidate information from radio base station 202.
In this case, search result information is sent only when the quality of radio communication is below a predetermined threshold. Thereby, mobile station 205 sends the search result information to a radio base station having a low quality of radio communication, and communication may be disconnected while connection candidate information is being received after sending the search result information. For example, when mobile station 205 is blocked by a metal door, etc., the quality of radio communication may get worse by opening/closing the door, thereby causing to be disconnected disconnect communication before finishing.
Accordingly, the foregoing technology capable of performing handover has a problem in that attempts to switch radio base stations frequently fail.